1. Field of the Invention
The present invention relates to a device for hydraulically controlling a continuously variable transmission having a V-belt by adjusting the widths of grooves of input and output pulleys depending upon the engine torque. More particularly, the invention relates to a device for hydraulically controlling a continuously variable transmission, which improves fuel efficiency, automatically detects the slipping state of the V-belt and suppresses the slip.
2. Prior Art
There has heretofore been known a continuously variable transmission which variably sets a transmission gear ratio between the input and the output by stretching a V-belt between a pair of pulleys and adjusting the widths of grooves of the pulleys round which the V-belt is wrapped.
In the continuously variable transmission of this kind as disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 42147/1988 and 272569/1992, movable conical plates provided for the pulleys are displaced by a hydraulic mechanism to vary the tension of the V-belt and the widths of grooves of the pulleys.
FIG. 7 is a diagram schematically illustrating the constitution of a general device for hydraulically controlling a V-belt type continuously variable transmission.
In FIG. 7, though widely known constitutional portions are not diagramed, an engine is equipped with an ignition device, and an intake pipe of the engine is equipped with a throttle valve and fuel injection valves.
The engine 1 and various actuators are equipped with various sensors (not shown) for detecting the operation conditions. Signals of various sensors are input to a controller 20 constituted by an ECU (electronic control unit).
A torque converter 3 having a damper clutch 2 is connected to the output side of the engine 1, and a back-and-forth change-over clutch is connected to the output side of the torque converter 3.
A CVT (continuously variable transmission) 5 is connected to the output side of the back-and-forth change-over clutch 4, and tires 7 of an automobile are connected to the output side of the CVT 5 via a differential gear 6.
The CVT 5 includes a first pulley 5a on the input side, a second pulley 5b on the output side, a V-belt 5c wrapped round between the first pulley 5a and the second pulley 5b, and hydraulic chambers 51 and 52 for adjusting the positions of the first pulley 5a and the second pulley 5b in the directions of arrows.
An oil pump 8 coupled to the engine 1 supplies an oil to the lubrication system of the engine 1 as well as to the hydraulic chambers 51 and 52 in the CVT 5 to adjust the CVT 5.
A conduit communicated with the oil pump 8 is provided with a flow rate control valve 9 for controlling a first hydraulic pressure (primary pressure) supplied to the hydraulic chamber 51 and a pressure control valve 10 for controlling a secondary hydraulic pressure (line pressure) supplied to the hydraulic chamber 52.
A conduit communicated with the hydraulic chamber 51 in the CVT 5 is provided with a primary pressure sensor 11 for detecting the primary pressure P1, a conduit communicated with the hydraulic chamber 52 in the CVT 5 is provided with a line pressure sensor 12 for detecting the line pressure P2, and the detected pressures P1 and P2 are input to the controller 20 like other various sensor signals.
The damper clutch 2 is provided with a direct-coupling duty solenoid 13, and the back-and-forth change-over clutch 4 is provided with a clutch duty solenoid 14.
Further, the flow rate control valve 9 is provided with a speed-change duty solenoid 15, and the pressure control valve 10 is provided with a line-pressure duty solenoid 16.
The duty solenoids 13 to 16 drive the damper clutch 2, back-and-forth change-over clutch 4, flow rate control valve 9 and pressure control valve 10 depending upon the control quantities from the controller 20.
In controlling the CVT 5, for example, the speed-change duty solenoid 15 drives the flow rate control valve 9 depending upon the primary pressure control amount C1 (hereinafter simply referred to as xe2x80x9ccontrol amountxe2x80x9d), and the line-pressure duty solenoid 16 drives the pressure control valve 10 depending upon the line pressure control amount C2 (hereinafter simply referred to as xe2x80x9ccontrol amountxe2x80x9d).
The shafts on the output side of the engine 1 is provided with first to third rotation sensors 17 to 19 for detecting the first to third rotational speeds N1 to N3. The detected rotational speeds N1 to N3 are input to the controller 20 like other various sensor signals.
The first rotation sensor 17 is provided between the torque converter 3 and the back-and-forth changeover clutch 4, the second rotation sensor 18 is provided between the back-and-forth change-over clutch 4 and the CVT 5, and the third rotation sensor 19 is provided between the CVT 5 and the differential gear 6.
Here, the second and third rotational speeds N2 and N3 stand for an input rotational speed and an output rotational speed of the CVT 5.
The controller 20 controls the primary pressure P1 and the line pressure P2 (first and second hydraulic pressures) based upon the operation conditions of the engine 1, input and output rotational speeds N2 and N3 of the CVT 5, and detected values of the primary pressure P1 and the line pressure P2 (first and second real hydraulic pressures).
In FIG. 7, the driving force produced by the engine 1 is, first, transmitted to the CVT 5 via the torque converter 3 and the back-and-forth change-over clutch 4.
At this moment, the back-and-forth change-over clutch 4 is changed over to forward, neutral or reverse by the clutch duty solenoid 14.
The CVT 5 controls the transmission gear ratio relying upon the first pulley 5a, second pulley 5b and belt 5c, and transmits the output torque from the second pulley 5b to the tires 7 through the differential gear 6.
The hydraulic pressure produced by the oil pump 8 is adjusted by the pressure control valve 10 and is supplied, as the line pressure P2, to the hydraulic chamber 52 of the second pulley 5b. 
Here, the pressure control valve 10 is controlled by the line-pressure duty solenoid 16 that is driven depending upon the control amount C2.
Further, the line pressure P2 adjusted by the pressure control valve 10 is divided by the flow rate control valve 9 and is supplied, as the primary pressure P1, to the hydraulic chamber 51 of the first pulley 5a. 
At this moment, the flow rate control valve 9 is controlled by the speed-change duty solenoid 15 that is driven depending upon the control amount C1.
Thus, the primary pressure P1 and the line pressure P2 are adjusted to adjust the positions of the pulleys 5a and 5b, and the transmission gear ratio is set to a target value by the tension of the V-belt 5c and the CVT 5.
FIG. 8 is a functional block diagram illustrating the constitution of the controller 20 in a conventional device for hydraulically controlling a continuously variable transmission, and shows an operation unit for determining a control amount C2 for the line-pressure duty solenoid.
The controller 20 includes a CVT input torque detector unit 21 for detecting the torque Ti input to the CVT 5, a CVT transmission-gear-ratio detector unit 22 for detecting the transmission gear ratio GR of the CVT 5, a target line pressure operation unit 23 for operating a target line pressure Po2, and a PID operation unit 24 for operating the control amount C2 of the line-pressure duty solenoid 16.
The CVT transmission-gear-ratio detector unit 22 operates the real transmission gear ratio GR based on the second rotational speed N2 (input rotational speed of the CVT 5) detected by the second rotation sensor 18 and the third rotational speed N3 (output rotational speed of the CVT 5) detected by the third rotation sensor 19.
The target line pressure operation unit 23 operates a target line pressure Po2 based on the torque Ti input to the CVT 5 and the transmission gear ratio GR.
The target line pressure Po2 corresponds to a hydraulic pressure (second hydraulic pressure) necessary for reliably clamping the V-belt 5c to the first and second pulleys 5a and 5b. 
The PID operation unit 24 operates, as the control amount C2, a PID correction quantity which is based on a line pressure difference xcex94P2 (=Po2xe2x88x92P2) between the target line pressure Po2 and the real line pressure P2 detected by the line pressure sensor 12.
That is, the PID operation unit 24 executes the PID control operation so that the real line pressure P2 is brought into agreement with the target line pressure Po2 based upon the data (input torque Ti and transmission gear ratio GR) of the CVT 5 to determine the control amount C2 of the line-pressure duty solenoid 16.
In the conventional device for hydraulically controlling the continuously variable transmission constituted as shown in FIGS. 7 and 8, when it is attempted to maintain a response of hydraulic pressure for the control quantities C1, C2 of the duty solenoids 15, 16 and to prevent slip of the V-belt 5c under every condition inclusive of after the aging, then, it becomes necessary to set a target line pressure Po2 which includes an excess of margin for the hydraulic pressure that is really necessary.
On the other hand, when the hydraulic margin is set to be small from the standpoint of improving fuel efficiency, then, the V-belt 5c may undergo the slipping due to the lack of line pressure P2 in case the control system of the line pressure P2 becomes no longer capable of following the change such as change in the vehicle state that occurs when the torque Ti input to the CVT 5 sharply increases.
Further, when an excessively large target line pressure Po2 is set to prevent the V-belt 5c from slipping, then, the oil pump 8 could become a large load to the engine to maintain a high line pressure P2 at all times, deteriorating the fuel efficiency.
According to the conventional device for hydraulically controlling the continuously variable transmission in which the line pressure P2 is maintained to be higher than the required pressure at all times, as described above, the line pressure P2 becomes insufficient in case the control system of the line pressure P2 becomes no longer capable of following the change, resulting in the slipping of the V-belt 5c. Besides, the oil pump 8 could become a large load upon the engine to deteriorate the fuel efficiency.
The present invention was accomplished in order to solve the above-mentioned problem, and its object is to provide a device for hydraulically controlling a continuously variable transmission which sets the line pressure to a minimum required pressure until the slip of a V-belt in the CVT is detected to lower the engine load and, hence, to improve the fuel efficiency and which, when the slip of the V-belt is detected, executes a correction processing for suppressing the slip to prevent the slip.
A device for hydraulically controlling a continuously variable transmission of the present invention comprises:
a continuously variable transmission of the V-belt type connected to the output side of an engine;
rotation sensors for detecting the input and output rotational speeds of the continuously variable transmission;
hydraulic pressure sensors for detecting first and second real hydraulic pressures for the first and second pulleys in the continuously variable transmission; and
a controller for controlling hydraulic pressures for said first and second pulleys based upon the operation conditions of said engine, upon the input and output rotational speeds of said continuously variable transmission, and upon said first and second real hydraulic pressures; wherein
the controller includes:
a belt slip detector unit which forms a slip detection signal when a slipping state of the V-belt is detected; and
a belt slip suppressing unit which executes a correction processing for suppressing the slip of the V-belt in response to the slip detection signal.
In the device for hydraulically controlling a continuously variable transmission of the invention, the controller includes:
a real transmission gear ratio operation unit for operating a real transmission gear ratio of the continuously variable transmission based upon the input and output rotational speeds; and
a virtual transmission gear ratio operation unit for operating a virtual transmission gear ratio of the continuously variable transmission based upon the real transmission gear ratio;
wherein the belt slip detector unit forms the slip detection signal based upon the comparison of the real transmission gear ratio with the virtual transmission gear ratio.
In the device for hydraulically controlling a continuously variable transmission of the invention, the belt slip detector unit forms the slip detection signal when a difference in the transmission gear ratio between the real transmission gear ratio and the virtual transmission gear ratio, becomes larger than a second predetermined amount in the direction of a second polarity opposite to the direction of a first polarity within a predetermined period of time from a moment when it became larger than a first predetermined amount in the direction of the first polarity.
In the device for hydraulically controlling a continuously variable transmission of the invention, the first and second predetermined amounts are set to be larger than a difference in the transmission gear ratio that occurs when normally changing the speed, and said predetermined period of time is set to be shorter than a time of from when a difference in the transmission gear ratio becomes larger than said first predetermined amount while normally changing the speed until when a difference in the transmission gear ratio becomes larger than said second predetermined amount.
In the device for hydraulically controlling a continuously variable transmission of the invention, the virtual transmission gear ratio operation unit operates the virtual transmission gear ratio by subjecting the real transmission gear ratio to the primary delay filtering, and a filter constant used in the primary delay processing is set to a value that does not change following the change in the real transmission gear ratio when the slip has occurred.
In the device for hydraulically controlling a continuously variable transmission of the invention, the controller includes a target line pressure operation unit that operates, as a target line pressure, the second hydraulic pressure necessary for clamping the V-belt to the first and second pulleys, and the belt slip detector unit validates the processing for detecting the slipping state when a difference in the line pressure between the target line pressure and the second real hydraulic pressure is greater than a predetermined amount.
In the device for hydraulically controlling a continuously variable transmission of the invention, the controller includes a target primary pressure operation unit for operating, as a target primary pressure, the first hydraulic pressure necessary for clamping the V-belt to the first and second pulleys, and the belt slip detector unit validates the processing for detecting the slipping state when a difference in the line pressure between the target line pressure and the first real hydraulic pressure is greater than a predetermined amount.
In the device for hydraulically controlling a continuously variable transmission of the invention, the belt slip suppressing unit adds a predetermined correction amount to the target line pressure in response to the slip detection signal.
In the device for hydraulically controlling a continuously variable transmission of the invention, the belt slip suppressing unit increases the correction amount by a predetermined amount every time when the slip detection signal is formed repetitively.
In the device for hydraulically controlling a continuously variable transmission of the invention, the controller includes an output torque control unit for controlling the output torque of the engine, and the belt slip suppressing unit decreases the output torque of the engine in response to the slip detection signal.